A valve for controlling low-temperature fluids typically has an upper or outer housing assembly including an outer housing member, a tube fixed to the member and having an end remote from the member, and an end collar welded on the tube end and a lower or inner housing member formed of high-alloy steel with an inlet port and an outlet port and a valve seat between the ports. The end collar is seated in the inner housing member and a valve stem having an outer end at the outer housing member and an inner end projects into the inner housing member and is guided in the end collar, extends through the tube between the members, and carries a valve body sealingly engageable with the seat. A connector is engaged around and fitted snugly to the end collar to secure these two parts together. Normally at least the end collar is made of a high alloy steel, typically a chromium/nickel stainless steel, and the inner member is made of a nonferrous metal, typically an aluminum alloy with magnesium and manganese.
Such valves are often used in very low-temperature applications, such as in the liquefication of helium or the rectification of air into hydrogen and oxygen. The purpose of the long connecting tube and valve stem is to hold the very cold parts as far as possible from the outer housing member, which typically has an operating handle or the like. The stem is typically tubular so that any leakage into the sleeve will form a vapor that will provide some thermal isolation.
In the known embodiments the end collar of the tube is formed with a flange forming part of the connector. A seal ring is engaged between this flange and the inner housing member and the collar and inner housing member are formed with complementary screwthreads to engage these two parts together and compress the seal ring. Such a connector comprised of screwthreads, a flange, and a seal ring is necessary because the collar is formed of a material that cannot readily be welded to the material of the inner member, even though a weld would be a much stronger and better connection. It is impossible to make a sufficiently strong tube and end collar of the same nonferrous metal needed for the inner member.
The problem with the complex seal-type connector is that with time the seal develops a leak. In particular the cold-temperature use means that this seal typically shrinks drastically and is generally subjected to enormous thermal stresses so its service life is fairly limited.